Some patients wait years for a definitive diagnosis. Using a unique combination of scientific and medical expertise and resources at the National Institutes of Health (NIH), the Undiagnosed Diseases Program pursues two goals: * To provide answers to patients with mysterious conditions that have long eluded diagnosis * To advance medical knowledge about rare and common diseases The program is trans-NIH in scope. It is organized by the National Human Genome Research Institute (NHGRI), the NIH Office of Rare Diseases Research (ORDR) and the NIH Clinical Center. Many medical specialties from other NIH research centers and institutes will contribute expertise needed to conduct the program, including endocrinology, immunology, oncology, dermatology, dentistry, cardiology, and genetics, which are represented among the dozens of participating senior attending physicians who may participate in the program's clinical research. Any longstanding medical condition that eludes diagnosis by a referring physician can be considered undiagnosed and may be of interest to this clinical research program. Of the total number of cases that may be referred to this program, a very limited number will be invited to proceed in the study at the discretion of the program[unreadable]s medical team.